The input and output ports of most color video equipment usually, by convention, have a nominal impedance of 75 ohms to match shielded 75 ohm co-axial cable.
Relatively recently there has been a move to standardize voice, data and other communication links on a common form of cable. Some systems currently use a form of cable known as unshielded twisted pair (UTP) while others are attempting to standardize on another form known as shielded twisted pair (STP), but both are essentially balanced lines having an impedance of around 100 ohms. While it is a relatively simple matter, by using suitable matching networks, to pass voice and data signals along such cable for considerable distances without deterioration of the signal, the passage of video signals, and particularly color video signals, has presented an obstacle which hitherto has proved insurmountable.
In order to pass color video signals along, say, UTP cable without degradation, an impedance matching device is required at each end of the cable in order to match the 75 ohm ports or co-axial line to the 100 ohm UTP. This does not present a particular problem in the case of some components of a multi-component video signal, most commonly the red and blue components, since a simple wide band impedance matching transformer can be used with reasonable results. On the other hand, major difficulties have been encountered in matching one component of the color signal (often the green component or, in some systems, a fourth component which carries timing information) because of its extremely wide bandwidth. By way of example, in one commonly used system the green component may include 55 Hz square wave vertical synchronization pulses, 26 KHz horizontal synchronization pulses, and picture information which is centred around 8 MHz and 16 MHz. Other current systems may require an even greater bandwidth, sometimes up to 100 MHz or more. In addition, it may be desirable that the network should allow the passage of d.c. in order, for example, that simple continuity tests can be carried out.
An aim of the present invention may be viewed as being to provide an arrangement which is capable of matching different input and output impedances, can be made to operate over a wide bandwidth (e.g. below 55 Hz to around 120 MHz for a color video signal), and has a low insertion loss.